A flow rate of a fluid flowing within a channel may be determined by placing a detector in the channel, heating the detector to a temperature above ambient, and measuring the extent to which heat is carried away from the detector by the fluid flowing past the detector. As the fluid flow rate increases, the amount of heat carried away increases, and the detector varies, e.g. in electrical resistance, in accordance with its temperature, thereby providing information about the heat carried away, and thus the fluid flow rate.
A heating element may heat the detector above ambient, and then, a small sensing current may be sent through the detector to measure its resistance during fluid flow. Alternatively, the sensing current may be large enough to heat the detector above ambient. In either case fluid flow may be calculated from the resistance of the detector, e.g., by placing the detector in a leg of a Wheatstone bridge circuit and measuring the output of the circuit to determine resistance and fluid flow.
Fluid flow sensors that include a detector heated above ambient typically are referred to as hot wire sensors. Some sensors of this type literally include a xe2x80x9chot wirexe2x80x9d (e.g. a platinum wire) suspended in the fluid flow channel for use as a detector. A thermal-sensitive resistive element deposited on a silicon substrate may also be used as a detector. Alternatively, a transistor or other element with a thermal-sensitive property, or other property for sensing fluid flow, similarly may be used.
As will be appreciated, the accuracy and response time of a thermal-sensitive element on a substrate typically depends in part on the thermal coupling between the element and the substrate. This is the case because the substrate may sink heat away from, or add heat to the thermal-sensitive element, depending on the substrate temperature. Substrate temperature, in turn, typically depends on other elements on the substrate, and/or on thermal coupling of the substrate to other devices (such as the hardware mounting of the substrate in the channel). In either case, the substrate typically provides a thermal mass much larger than that of the thermal-sensitive element, and thus tends to slow the response of the thermal-sensitive element to changes in fluid flow.
Furthermore, it will be understood that a hot wire sensor typically senses fluid flow at a particular position in the flow channel, or in the case of an actual wire, along a line defined across the flow channel. The sensor or wire then may provide a single data point, which is used to calculate the flow rate for the entire channel. Unfortunately, hot wire sensors may include inherent inaccuracies, such as non-linear dependence on temperature, process variations between hot wire sensors of the same type, and/or degradation due to aging or contaminant deposits. Furthermore, flow rate typically varies across the flow channel, remaining at zero within a boundary layer that forms at the wall of the channel and reaching a maximum in the free stream that is closer to the center of the flow channel. Accuracy of the calculated flow rate thus depends on the accuracy of the sensor, and also on the extent to which the particular position of the hot wire sensor represents fluid flow in the channel as a whole. Also, for an actual xe2x80x9chot wire,xe2x80x9d accuracy depends on the extent to which the wire is accurate in lumping a distributed measurement into a single data point.
Another limitation of hot wire sensors is that such sensors typically do not provide information about the direction of fluid flow in a channel. That is, a hot wire sensor is equally cooled by fluid flow in one direction through the channel as it is by fluid flow in the opposite direction. Fluid flow that alternates back and forth thus would typically result in a report of fluid flow in absolute terms rather than as an algebraic sum more representative of average fluid flow.
A flow sensor system is provided for measuring the rate of flow of a fluid in a flow channel that is configured to contain the fluid flow. A substrate may be disposed in the flow channel, and a plurality of transducers may be disposed on the substrate in the flow channel. The substrate may also provide a plurality of paths, and each path may conduct a signal. The transducers may be configured to respond to the fluid flow by modifying the signals in relation to the fluid flow. The transducers may be arranged with at least one of the transducers disposed closer to a central longitudinal axis of the flow channel than at least one other transducer. A signal processor may be coupled to at least two of the signals, and the signal processor may calculate the rate of flow of the fluid as a function of the at least two signals.